Silicon nitride ceramics have high strength and thermal shock resistance characteristics which make them ideal materials for such applications as ceramic engine parts, cutting tools, bearings, and the like. In order to make reliable and reproducible sintered silicon nitride components, the starting silicon nitride powders should be highly pure. Silicon nitride powders have been prepared using a direct nitridation method in which fine metallic silicon powder is heated at a temperature about 1250.degree. C. in a nitrogen or ammonia atmosphere for a prolonged period. Silicon nitride powder can also be prepared using a carbothermal process in which fine SiO.sub.2 powders are reduced with carbon and simultaneously nitrided with nitrogen or ammonia at temperatures higher than 1300.degree. C. In a gas phase synthesis, silicon halides are reacted with ammonia in a tubular reactor at temperatures as high as 1000.degree. C. to produce silicon nitride. Yet another gas phase process involves the thermal decomposition of organosilicon compounds to form amorphous powders. The as-reacted powders are then crystallized by heat treating at temperatures above 1200.degree. C.
In a thermal decomposition of diimide process, silicon halides are reacted with ammonia in a liquid phase at temperatures below 0.degree. C. The precipitates of this process are then converted to Si.sub.3 N.sub.4 powder by heat treating at temperatures up to 1500.degree. C.
The direct nitridation process not only requires a prolonged heat treatment, but also is difficult to control due to the large amount of energy released (about 5.2 kJ per gram of Si.sub.3 N.sub.4 formed at 1327.degree. C.) during nitridation. The processing time can be reduced by using Si powders with Fe impurities. However, this practice is not suitable for manufacturing high purity Si.sub.3 N.sub.4 powders. The carbothermal process requires an excess amount of carbon to facilitate the reduction of SiO.sub.2. Any remaining carbon in the product is then removed by oxidative treatments. Thus, Si.sub.3 N.sub.4 powders produced by the carbothermal process usually contain higher levels of carbon and/or oxygen. The use of reaction promoters, e.g., Mg and Ca compounds, also has negative effects on the purity of Si.sub.3 N.sub.4 powders.
The gas phase reaction using silicon halides and ammonia as starting materials has two major disadvantages: (i) the process equipment can be plugged up by NH.sub.4 Cl deposit, a condensible by-product; and (ii) the presence of residual chloride in the calcined Si.sub.3 N.sub.4 has unfavorable impacts on powder sinterability. The thermal decomposition of organosilicon compounds produces powders with substantial amounts of carbon, which needs to be eliminated by heat treating in a mild oxidative atmosphere. Thus, this process also suffers from carbon and/or oxygen impurities.
For the thermal decomposition of diimide processes, the formation of NH.sub.4 Cl precipitates can plug up the feed and exit pipes in the liquid phase reactor. The diimide precursor also needs to be dechlorinated by heat treating at temperatures up to 800.degree. C. before it can be calcined to produce crystalline Si.sub.3 N.sub.4 powders. The final Si.sub.3 N.sub.4 powders still may have up to 100 ppm of chloride.
U.S. Pat. No. 4,122,155 discloses a process for producing crystalline .alpha.-Si.sub.3 N.sub.4 powder free of metallic and non-metallic impurities except for oxygen, which consists essentially of reacting a gaseous mixture of silane and anhydrous ammonia at a reaction temperature ranging from about 600.degree. C. to about 1000.degree. C. to produce an amorphous powdery reaction product in an amount of at least 90% of theoretical yield based on silane, said anhydrous ammonia being used in an amount ranging from about 15 times to about 25 times in excess of the stoichiometric amount, calcining said amorphous powdery reaction product at a temperature ranging from about 1400.degree. C. to 1600.degree. C. for a period of time sufficient to convert at least a substantial amount thereof to crystalline .alpha.-Si.sub.3 N.sub.4 powder ranging in surface area from about 5 m.sup.2 /g to about 15 m.sup.2 /g, said calcining being carried out in an atmosphere of nitrogen.
It is an object of the present invention to provide an economical process for producing high purity silicon nitride powders.
It is another object of the present invention to provide a process for producing high purity .alpha.-Si.sub.3 N.sub.4 powders from the reaction between ammonia and silane in a molar ratio of 7:1 and higher.
It is another object of the present invention to provide a process for producing high purity .alpha.-Si.sub.3 N.sub.4 powders that meet the specifications of structural ceramic grade silicon nitride.
The foregoing and additional objects will become fully apparent from the following description.